The present invention relates to devices for evaluating the density of protons present in a given body, making it possible to identify the material forming part of the constitution of said body and to which the protons belong, which devices find particularly advantageous applications in particular in the field of the food industry in order to determine the quantity of water and/or fat in foodstuffs, and in the field of the oil industry in order to determine, for example, the quantity of water in the rock surrounding the wall of a borehole, the distribution of pores within said rock, and/or its permeability.
There already exists a technique known as nuclear magnetic resonance (NMR) for determining the quantity of protons in a given material, i.e. the number of hydrogen free radicals, and possibly also for identifying said material. Numerous devices have been made for implementing that technique. Very briefly, those devices comprise means for inducing a static magnetic field in a given zone of the material to be investigated, with the lines of force of the field having a given direction for polarizing the protons present in said zone. Such means are generally constituted by a permanent magnet.
The device also comprises a transmitter antenna constituted for example by a magnetic coil powered by a radio frequency electricity generator, the coil being associated with the permanent magnet in such a manner that the lines of force of the radio frequency magnetic field induced in said zone by said coil when powered by radio frequency electrical current make a non-zero angle with the lines of force of the static field, and advantageously make an angle equal to ninety degrees.
The frequency of the radio frequency field is such that it corresponds to the precession frequency of protons in the static field so as to induce resonant interaction between the protons and the radio frequency field. This interaction makes it possible by means of the radio frequency field pulses to turn the polarization of the protons through an angle that depends on the amplitude of the radio frequency field and on the duration of the pulses.
The device also comprises a receiver antenna constituted for example by a magnetic coil which may be constituted by the same coil as the transmitter coil, the receiver coil picking up the magnetic field produced by the protons once the radio frequency current has been switched off and while the protons are returning to their initial polarization under the sole action of the static field of the permanent magnet.
The amplitude of the magnetic field detected by the receiver antenna is a function of the number of protons that have been excited in the investigation zone, and the time taken by the protons to switch from their second polarization back to the first, known in the art as the xe2x80x9crelaxation timexe2x80x9d, is an image of the nature of the substances to which the protons belong.
That technique is well known to the person skilled in the art and is described in numerous documents, so it is not described in greater detail herein.
There exist numerous devices enabling that technique to be implemented, and that satisfy the above description, for example the device described in U.S. Pat. No. 5,610,522.
Nevertheless, all presently-known devices of that type either present a structure that is very complex, or else they lack power, or else they cannot be applied to fields that are as different as the food industry and the oil industry.
An object of the present invention is thus to provide a device for evaluating the density of protons in a material contained in a given body, with the possibility of identifying said material, which device mitigates to a considerable extent the drawbacks of prior art devices and can be applied to fields that are as different as the food industry and the oil industry.
More precisely, the present invention provides a device for evaluating the density of protons present in a given body with the possibility of identifying the material forming part of the constitution of the body and to which the protons belong, the device being characterized by the fact that it comprises:
a permanent magnet having two poles, respectively a north pole and a south pole, said permanent magnet presenting a longitudinal axis of symmetry oriented north-south and a plane of symmetry perpendicular to said longitudinal axis of symmetry;
at least first and second cylindrical coils disposed at opposite ends of the permanent magnet, substantially centered on said longitudinal axis of symmetry, and substantially symmetrical about said plane of symmetry;
a controllable radio frequency electricity generator having outputs respectively connected to the first and second coils so that the two coils are fed at radio frequency so that the faces of the two coils respectively facing the two poles of the permanent magnet are of the same magnetic kind; and
a receiver having inputs respectively connected to said two coils, said receiver being suitable for analyzing the electrical signals delivered at the output of the two coils when the generator is disconnected from said two coils.